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Simulation Study of Energy Chirp Induced Effects in Laser Wakefield Accelerator Driven Free Electron Laser

Beam energy compression via chicane magnets has been proved to be an effective method to reduce the slice energy spread of electron beams generated by laser wakefield accelerators (LWFAs). This technique has been widely adopted by leading research teams in experiments targeting future compact, high-gain free electron lasers (FELs). However, after energy compression, a strong beam energy chirp is introduced into the electron beam, which substantially hinders the microbunching process and impairs spectral coherence. Here, we present a detailed, unaveraged three-dimensional simulation that examines the effects of this energy chirp, and the results can be applied to the design of a proposed LWFA-driven VUV FEL. The energy chirp in a LWFA-produced electron beam causes FEL interactions at multiple resonant frequencies across the entire electron bunch, simultaneously, which prevents sustained radiation power growth at the designed frequency along the undulator. Consequently, spectral purity is significantly degraded. Additionally, due to undulator dispersion, the energy chirp leads to an elongation of the bunch length, which increases microbunch separation. This results in a noticeable redshift in the radiation frequency and further disruption of spectral purity. These effects are compared to the ideal scenario in which the energy chirp is removed following energy compression. Simulation results indicate that the implementation of a beam dechirper is a crucial step for improving the saturation of radiation power. Insights gained from this simulation of energy chirp-induced mechanisms will aid in the development of more effective compensation strategies, ultimately optimizing LWFA-driven FEL designs.